The present invention relates to a master cylinder used in a vehicle hydraulic brake system, and specifically a master cylinder including a piston which is in sliding contact with the inner peripheral surfaces of cups held on a cylinder body to pressurize brake fluid in a pressure chamber defined in the cylinder body. (Such a master cylinder is hereinafter referred to as a cup guide type master cylinder.)
Many of today's vehicle hydraulic brake systems have one or more automatic brake control functions which are initiated by an electronic control unit while the brake pedal is not pushed in, such as a vehicle stability control function.
Unexamined JP patent publication 2000-71969 discloses a cup guide type master cylinder used in this type of hydraulic brake system.
The master cylinder disclosed in this publication includes a spacer disposed between a primary cup surrounding the piston and a guide provided in the rear of the primary cup. A groove is formed rearwardly of the spacer through which brake fluid is supplied from a reservoir into a pressure chamber of the master cylinder.
It is required that a cup guide type master cylinder used for automatic brake control be capable of smoothly sucking brake fluid from the reservoir into the pressure chamber not only when the piston returns to its original, inoperative position after it has been advanced, but also during automatic brake control.
In order to fulfill both of these requirements, piston ports (ports formed in the piston) through which the pressure chamber communicates with the reservoir have to be positioned as close to the primary cup as possible when the brake pedal is not being operated and thus the piston is in its initial, inoperative position. For this purpose, the abovementioned spacer has to be as thin as possible.
When the piston returns, brake fluid is sucked from the reservoir into the pressure chamber through a gap defined around the primary cup because the primary cup is deformed under negative pressure produced in the pressure chamber. During automatic brake control, brake fluid is sucked into the pressure chamber through the piston ports.
In the arrangement of the above JP patent publication, hydraulic pressure tends to act on the spacer through the primary cup. The spacer thus tends to be subjected to large stresses, which may cause premature fatigue failure of the spacer.
In order to further smoothly suck brake fluid from the reservoir into the pressure chamber during automatic brake control, the back of the spacer has to be positioned further close to the piston ports. For this purpose, the thickness of the spacer has to be further reduced. This is however difficult because the groove is formed behind the spacer, so that the spacer has to be supported by a plurality of ribs. If the thickness of such a spacer is further reduced, its strength will be insufficient.
Further, because there exist diametric gaps between the spacer and the cylinder body and between the spacer and the piston, the spacer tends to move radially so as to be eccentric to the cylinder body. This impairs uniform flow of brake fluid through these gaps. Specifically, when the piston is returning toward its original position, brake fluid may not smoothly flow through the gap defined around the primary cup into the pressure chamber. While the piston is in its original position, brake fluid may not smoothly flow through the gap defined between the spacer and the piston and the piston ports into the pressure chamber.
Further, due to eccentric movements of the spacer relative to the cylinder body, the gaps between the spacer and the cylinder body and between the spacer and the piston tend to locally increase in radial dimension. Under hydraulic pressure, the primary cup may wedge into such a radially expanded portion of one of the gaps and get damaged.
The spacer and the guide have to be held in position by screwing a cover after mounting the spacer and the guide. Thus, this master cylinder is complicated in structure and needs a large number of assembling steps.
An object of the present invention is to provide a master cylinder of the abovementioned type which is free of any of the problems mentioned above, specifically, a master cylinder which is simple in structure but reliable, and which ensures smooth suction of brake fluid into the pressure chamber while the piston is returning and during automatic brake control.